During a semiconductor fabrication process semiconductor dice are formed on a wafer. The wafer includes a semiconducting substrate, such as silicon or gallium arsenide, on which integrated circuits are formed. During and following the fabrication process, the wafer must be tested to evaluate the electrical characteristics of the integrated circuits. For example, standard wafer probe is performed to test the gross functionality of each die contained on the wafer. Speed probe is performed to test the speed performance of the dice. Other tests such as full functionality testing, burn-in testing and dynamic burn-in testing are typically performed after the dice have been singulated from the wafer.
Currently semiconductor wafers are probe tested utilizing probe cards. One type of probe card includes needle probes for making temporary electrical connections between contact locations on the dice (e.g., bond pads, fuse pads, test pads) and external test circuitry. The probe card typically includes an insulating substrate, such as a glass filled plastic. The substrate can include electrical traces in electrical communication with the needle probes. In addition, the needle probes can be configured to contact a specific die on the wafer. Typically the wafer or the probe card is stepped so that the dice on the wafer are tested one at a time in sequence. There are also probe cards configured to test multiple dice on the wafer (e.g., 8 or 16).
One problem with this type of conventional probe card is that the planarity and positions of the needle probes can vary. Typically the needle probes must initially be bent by hand into required x and y locations to match the contact locations on the wafer. This is a very labor intensive procedure. Also the z-direction location of the contact locations on the wafer can vary. This can cause inaccuracies in the test results because electrical contact with the different contact locations can vary. Continued use of the needle probes causes deformation and further misalignment of the needle probes. Probe cards with needle probes are thus expensive to fabricate and expensive to maintain.
Another problem with needle probe cards is that the contact locations on the wafer are typically coated with a metal oxide layer. For example, aluminum test pads can be covered with aluminum oxide that forms by oxidation of the underlying metal. This metal oxide is electrically non-conductive and provides a high degree of electrical resistance to the needle probes. In order to ensure accurate test results, the needle probes must penetrate this oxide layer to the underlying metal film. This requires "overdrive" and "scrub" forces which can damage the contact locations and wafer.
Typically, to penetrate the oxide, the probe card and wafer are brought together until the needle probes contact the contact location. The probe card is then "overdriven" a distance (e.g., 3 mils) which deflects the needle probes and causes them to bend. As the needle probes bend, the ends of the needle probes move horizontally across the contact location causing the ends to scrape over the metal. This causes the ends to break through the native oxide layer and contact the underlying metal. The scrubbing action also displaces some of the metal on the contact location causing a groove and a corresponding ridge.
Still another problem with needle probe cards is that they require the contact locations on the dice to be oversized. In particular due to the inaccuracies in the x-y placement of the needle probes, the contact locations on the dice must made large enough to accommodate alignment variations between the needle probes. This requires that the contact locations be made larger by default, which in turn makes the dice larger.
To overcome some of the problems associated with conventional needle probes, membrane probe cards have been developed. Membrane probe cards are manufactured by Packard Hughes Interconnect of Los Angeles, Calif., and others. Membrane probe cards typically include a membrane formed of a thin and flexible dielectric material such as polyimide. Contact bumps are formed on the membrane in electrical communication with conductive traces, typically formed of copper. The conductive traces electrically connect to external test circuitry.
In general, membrane probes are able to compensate for vertical misalignment between the contact locations on the wafer. In addition, the membrane probe can include a force applying mechanism that allows the contact bumps to penetrate the oxide layer of the die contact locations. Membrane probes usually don't have the "scrubbing" action of needle probes. Rather membrane probes rely on penetration contact bumps to break through the oxide and contact the underlying metal.
One disadvantage of membrane probes is that vertical "overdrive" forces are required to penetrate the oxide and make a reliable electrical connection between the contact bumps on the probe and the contact locations on the wafer. These forces can damage the contact locations and the wafer. In addition, the contact bumps and membranes can be repeatedly stressed by the forces. These forces can also cause the membrane to lose its resiliency.
Another disadvantage of membrane probe card is the CTE (coefficient of thermal expansion) mismatch between the probe card and wafer. In particular the copper traces on the probe card will move as a result of temperature fluctuations causing the contact bumps to scrub across the contact locations. This can damage the contact locations or an associated passivation layer. In addition, the masks required to make the membranes are difficult to make with high volume processes. This makes membrane probe cards very expensive.
Yet another disadvantage of probe cards is that dynamic burn-in and full functionality testing are typically performed at the die level rather than at the wafer level. One reason that these test procedures are not performed at the wafer level is that these tests require a large number of connections with the wafer. In addition, a large number of input/output paths between the wafer and test circuitry are required. For example, a wafer can include several hundred dice each having twenty or more bond pads. The total number of bond pads on the wafer can be in the thousands. For some tests procedures an input/output path must be provided to each bond pad. Even with wafer stepping techniques, conventionally formed probe cards, usually do not include enough probes or contact bumps to test groups of dice having a large number of contact locations.
In view of the deficiencies of prior art probe cards, improved probe cards are needed for semiconductor manufacture.